IZANAGI-PACIFIC RIDGE SUBDUCTION IN NE ASIA IN THE EARLY CENOZOIC: A REVIEW OF GEOLOGICAL EVIDENCE FROM NE ASIA

The Izanagi-Pacific ridge arrived at the NE Asian margin at ca. 60 Ma and was finally subducted beneath the continental margin at ca. 50 Ma. However, the evidence on the continental side is absent. In this review, we summarize the new studies about magmatism, structural analysis of basins and faults, low-temperature geochronology and paleomagnetism in NE Asia from Cretaceous to Eocene.

The ridge subduction event in the NE Asian margin is presented by two major changes.

The first one is that the NNW-directed subduction of the Izanagi plate in the Cretaceous was replaced by the NWW- or W-directed subduction of the Pacific plate from the late Paleocene: 1) the major strike-slip faults changed from sinistral in the Cretaceous to dextral in the early Cenozoic; 2) abruptly increasing cooling rate in NE China during ca. 65 to 50 Ma and coeval regional angular unconformities in the basins, indicating the strengthened westward compression when the Pacific plate subduction initiated; 3) the plutons in the late Early Cretaceous were distributed along the Central Sikhote-Alin Fault zone with en echelon patterns, showing sinistral simple shear background. In the Paleocene to early Eocene, the alignments of calderas changed to roughly E-W trending, implying an E-W principle compression.

The second major change in the early Cenozoic Asian margin is the magmatism related to the open of a slab window and the ridge-trench interaction: 1) the Cretaceous supra-subduction arc magmatism was replaced by A-type, adakite-like granitoids, basaltic rocks with intra-plate infinities after a short magmatic gap during 60-50 Ma in Sikhote-Alin and SE Korea; 2) fore-arc MORB intrusions in coeval or lightly older accretionary prisms in Hokkaido and A- and S-type granitoids in Sakhalin; 3) the Sr-Nd-Hf isotopes of the magmatic rocks show dramatic increase of mantle-derived component after the magmatic gap (65-50 Ma) in Sikhote-Alin, Sakhalin and Japanese Islands.

Thus, the ridge subduction model is proposed in this paper to explain the important changes of tectono-thermal conditions in the NE Asian margin from the Cretaceous to Eocene. The slab window supplied channels for the hot asthenosphere, overprinting the arc-related geochemical and isotopic features. After the consumption of the Izanagi plate, the Pacific subduction began and the convergence direction changed to NWW or nearly W, forcing the Sikhote-Alin terranes rotated counterclockwise, enhancing the E-W compressive component, and the dextral strike-slip motion along the major NNE-trending faults in NE Asia.